Grafting Energy-Harvesting Leaves
onto the Sensornet Tree
AUTHORS:
Lohit Yervay, Bradford Campbelly, Apoorva Bansaly,
Thomas Schmidz, Prabal Duttay
Presenting by:
Phanindar Reddy Tati
Contents:
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Abstract
Introduction
System overview
Low-Power leaf node design
Evaluation
Related work
Conclusion
Abstract:
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We study the problem of augmenting battery-powered sensornet trees with energyharvesting leaf nodes. Our results show that leaf nodes that are smaller in size than today’s
typical battery-powered sensors can harvest enough energy from ambient sources to
acquire and transmit sensor readings every minute, even under poor lighting conditions.
However, achieving this functionality, especially as leaf nodes scale in size, requires new
platforms, protocols, and programming. Platforms must be designed around low-leakage
operation, offer a richer power supply control interface for system software, and employ an
unconventional energy storage hierarchy. Protocols must not only be low-power, but they
must also become low-energy, which affects initial and ongoing synchronization, and
periodic communications. Systems programming, and especially bootup and
communications, must become low-latency, by eliminating conservative timeouts and
startup dependencies, and embracing high-concurrency. Applying these principles, we show
that robust, indoor, perpetual sensing is viable using off-the-shelf technology.
Continued…
 Problem: Augmenting battery-powered sensornet trees with energyharvesting leaf nodes
 Results shows leaf nodes smaller in size works fine.
 Need new platforms, protocols and programming
 Platforms:
 Low leakage operation
 Offer richer power supply
 Employs energy storage hierarchy
 Protocols:
 Low-power and low-energy protocols
 Programming:
 Fast Boot-up
 Low-latency
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ABSTRACT
Introduction:
 Energy Harvesting operation
 Approaches:
 EnOcean
 ZigBee Green Power
 Energy harvesting and Mesh Networking are not exclusive , can exist in
a unified network architecture.
 New technologies coupled with Star topology addresses challenges in
energy harvesting operation.
 Existing technologies can be combined in new ways with simple
protocols to achieve energy harvesting operation.
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ABSTRACT
INTRODUCTION
Continued…
 Adding stable clock and minor software improvements to existing
battery powered mesh nodes prepares them to interact with energy
harvesting leaf nodes.
 Leaf nodes:
 Similar to Branch nodes
 No batteries
 Solar cells
 Design Constraints:
 Low-leakage
 Low-power operation
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ABSTRACT
INTRODUCTION
Continued…
 Networking problems in augmentation:
 Initial Synchronization
 Ongoing Synchronization
 Bi-directional communications
 Challenge: Achieve low-energy operation
 Positive side:
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low communication is possible
low-energy neighbor discovery protocols available
 Optimizations are required
 Goal: Understanding design space of low-maintenance, high-density
sensor networks
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ABSTRACT
INTRODUCTION
Continued…
 Authors Showed,
 With available parts, we can build solar-powered node with ultra low
leakage currents
 Node works in very low indoor lighting conditions
 Delivers data every minute
 Adapted existing protocols to meet challenges of the problem
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ABSTRACT
INTRODUCTION
System Overview:
 Elements:
 Wall-powered trunk nodes
 Battery powered Branch nodes
 Energy harvesting leaf nodes
 Leaf node:
 Integrates sensor node like ‘Epic mote’
 Energy harvesting power supply
 Accurate time keeping
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INTRODUCTION
SYSTEM OVERVIEW
Continued…
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INTRODUCTION
SYSTEM OVERVIEW
Continued…
 Five design principles in leaf nodes:.
 Minimize power transfer inefficiencies
 Minimize power conversion inefficiencies
 Minimize leakages
 Improve energy consumption efficiency
 Minimize communication cost
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INTRODUCTION
SYSTEM OVERVIEW
Continued…
 Branch Nodes:
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Battery and Regulator
Clock
Identical to sensors ‘Telos’
 Leaf to Branch Communications:
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Wakes up on a fixed period
Takes a sensor reading
Transmits packet
Listens inbound traffic
 What if a leaf node does not have power?
 Low duty cycle neighbor discovery protocol
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INTRODUCTION
SYSTEM OVERVIEW
Low-power leaf node design:
 Leaf Node:
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Processor
Radio
Real-time Clock
Energy Harvesting power supply
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SYSTEM OVERVIEW
LOW-POWER LEAF NODE DESIGN
Continued…
 Processor and Radio:
 Epic core mote
 MSP430F1611 microcontroller
 CC2420 Radio
 Real time Clock:
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To investigate leaf activity
NXP PCF2127A RTC
Excellent time keeping stability
Low current draw
Flexible triggering options
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SYSTEM OVERVIEW
LOW-POWER LEAF NODE DESIGN
Continued…
 Hardware Operation:
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Charge
Startup
Active
 Software Operation:
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Shutdown
Oscillator Fast Start
Optimized startup
Concurrent initializations
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SYSTEM OVERVIEW
LOW-POWER LEAF NODE DESIGN
Evaluation :
 Evaluates the viability of energy harvesting operation, characterizes typical
indoor lighting conditions, demonstrates initial and ongoing synchronization,
and shows that leaf and branch nodes can communicate successfully.
 Energy Harvesting Operation:
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Demonstrates relation between irradiance and leaf node activity
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Two Leaf nodes with Amorphous Solar cell
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Work: Transmit a packet and disconnect processor and radio from power supply
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Exposed to varying irradiance levels from 4 indoor light sources
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Question: Given certain level of radiance, what is the transmission rate of leaf node?
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Two Leaf nodes with Crystalline Solar cell
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
Amorphous
6.a
Crystalline
Similar conversion factors
under fluorescent spectrum
6.b & c
Exhibits more conversion in
incandescent and halogen
settings
6.d
Similar results under LED
6.e
Shows indoor locations are
viable to leaf nodes
6.f
Daily irradiation is fine for leaf
nodes
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
 Initial Synchronization:
 Two techniques to synchronize leaf nodes and branch nodes
 Asynchronous neighbor discovery
 Synchronous event triggered
 Asynchronous neighbor discovery:
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Disco neighbor discovery protocol
Leaf nodes transmits beacons, branch nodes listen
Worst case discovery latency = 50mins
Discovery burden is small for both nodes
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
 Leaf Nodes:
 Transmits beacons in a 5ms window, every 60s
 This allows a branch node to both employ a compatible neighbor
discovery schedule and predict future transmission times
 Branch Nodes:
 listens for beacons in 5ms window, every 245ms
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
 Synchronous event triggered:
 Designed a simple, zero-power, light activated trigger switch
 Leaf node transmits first and then branch node responds
 Transmission is bidirectional
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
 Ongoing Synchronization:
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More burden on Branch node
Leaf node simply transmits beacons every multiples of 60s
Branch node keeps track of leaf nodes transmission times
Example
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
 Ongoing Synchronization:
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Continued…
 Leaf to Branch Communication:
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Relatively straight forward
Transmits on multiples of 60s
If it does not have power, skips that activity cycle
 Branch to Leaf Communication:
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Like sending ACK to leaf node
To enable this, we modify the branch to pipeline payload reception with transmit FIFO
loading. This allows the branch node to reply with a full packet with a 0.67ms
turnaround time.
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LOW-POWER LEAF NODE DESIGN
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EVALUATION
Related Work:
 Architectures:
 The canonical sensornet is the Great Duck Island deployment.
 The battery-powered nodes generated a message every 5 minutes.
In this work, authors show that purely energy-harvesting indoor
nodes can send a message about every minute during daylight hours.
 Authors extended this architecture one level further, into energy
harvesting leaves, and describe how these leaves can interoperate
with the existing architectures.
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EVALUATION
RELATED WORK
Continued…
 Indoor Photo-Voltaic Systems:
 TwinStar is a mixed indoor-outdoor solar energy harvesting system
that explores a capacitor-only energy storage design.
 The idea behind TwinStar is to use energy when it is available, and
thus reduce energy leakage.
 Their design is practical with batteries, however, our work explores
the scenario in which complete energy loss is possible.
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EVALUATION
RELATED WORK
Conclusion:
 Batteries have a finite lifetime, they incur replacement costs, and their
average power delivery scales poorly compared with indoor
photovoltaic.
 Today many believe that energy-harvesting holds the key to long-term,
cost-effective, and sustainable sensing.
 This paper shows that it is possible to augment battery powered mesh
networks with energy-harvesting leaf nodes.
 Authors created a new tier of sensor nodes that are free from the
constraints of battery power, but still retain the many benefits of
interoperating with contemporary wireless multihop mesh networks.
 This work paves the way for a new tier of perpetual computing systems,
shows the viability of the architectural approach, and demonstrates
interoperability with existing sensor network nodes.
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RELATED WORK
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CONCLUSION